It has previously been proposed to make the center electrode on a spark plug by applying a noble-metal suspension on the facing end of the center insulator of the spark plug. An electrical connection is made to the interior of the insulator and a therein-contained electrically conductive composition, for example an electrically conductive ceramic mass which is located in a longitudinal bore of the insulator. The electrically conductive mass may also include suspension of the noble metal. A spark plug of this type is described in the referenced German Patent Disclosure Document No. DE-OS 31 32 903.
The spark plug of the type described is extremely effective; due to the use of the noble metal, however, the spark plug is expensive. The layer of noble metal suspension which is located at the ignition end portion of the spark plug, exposed to the combustion gases and to the spark from a counter electrode, must be comparatively thick in order to insure sufficient lifetime of the spark plug. If the layer is too thin - which saves noble metal, typically platinum, too much of the noble metal is burned of upon sparking of the spark plug and the lifetime of the spark plug is impaired.
The noble metal coating at the facing end of the insulator can be applied by melt electrolysis, galvanic electrolysis, and even by brazing or welding a thin noble metal sheet on the insulator. Regardless of form, however, the requirement for noble metals in such a spark plug is high, so that the cost of the spark plug is also commensurately high.